I have very much enjoyed reading the thread. Ultimately, it comes down to preferences, and really there's no right or wrong answer. Kurt Kamka, Dave Martinez, and Allen, among others, has shown how well modern glass can play with the MM's sensor.
From my standpoint, having a drawer full of modern glass and now classic lenses, I prefer how the classic lenses render, and as Roger mentioned, the 35 mm 8-element 'cron and 50 mm f/2 Rigid cron in particular. Therea a host of lovely 90ish mm lenses that work well on the MM in my eyes (90 'cron E48, Canon 100 mm f/2, Canon 85 mm f/1.8, Nikkor 85 mm f/2, to name a few).
What I have found, and what very much suprised me, was that the older glass (in particular, the Rigid cron) renders equisitie sharpness (just as good as modern glass) in the center part of the frame across its aperture range (particularly when stopped down a bit, but wide open is VERY acceptable), while providing a nice OOF rendition/bokeh/whathave you....Further, there's less inherent macrocontrast in these older lenses, which in many cases were coated with BW film in mind, from what I have gathered. This provides a flatter raw file that is then even more malleable for post processing in the most pleasant ways.
What I found with modern glass (granted, my subjective experience) is that the look on the MM tends to look very clean, polished, clinical, and SHARP (maybe too sharp), and my images have tended to look a bit "HDR"ish when using this glass. Using the older glass, at least for me, as provided a more pleasant look in keeping with my BW aesthetic. There's nothing really wrong with acheiving either look, but I found that I have enjoyed the look coming from my classic lenses more, due to the interesting OOF chracteristics, advanced tonal greyscale range, and forgiving sharpness....
All in all, the MM is my favorite digital camera ever. I have nearly forgone color as a result, expect for when I play with the RX1R and M9....My modern lenses have gotten less use...it may be my journey in enjoying classic lenses, but it sure has been enjoyable...by the way, I have many more goodies to share
....images, that is